“That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all,” -Romans 4:16
Paul is wrapping up his argument now. He summarizes that if the promise is to be guaranteed to all of Abraham’s offspring, then it must rest on grace (unmerited favor); therefore, to that end, God has determined that the fulfillment of the promise should come through faith.
In other words, the promise only stands firm when it depends on grace because that is the same as depending on God. John Calvin noted, “that as long as men depend on works, they are harassed with doubts.” Thus, if they are harassed with doubts (which is not faith) then they deprive themselves of what the promises contain.